In the typical tri-cone roller cutter bit, the cutter elements are mounted on the outer surface of three hollow, cone-shaped cutters. Each of these three cone-shaped cutters is mounted on a respective cylindrical bearing pin which in turn extends down from the lower end of a lug. The three lugs are welded together to make a complete bit.
The three bearing pins extend down from their respective lugs toward the longitudinal axis of the bit, with the axis of each bearing pin sloping downwardly at a selected angle as shown in FIG. 1 in the general direction of the axis of the bit. Each bearing pin typically has two bearing journals, one formed on an outermost or upper cylindrical section of larger diameter, and the other formed on an innermost or lower cylindrical section of smaller diameter. Each of these journals can have a bearing race upon which a plurality of roller bearings ride. These bearings provide lateral support between the bearing pin and the cone-shaped cutter. Axial support is provided by thrust-absorbing means such as a tungsten carbide thrust button which is typically mounted on the lower end of the bearing pin and such as thrust bearing shoulders between the two bearing journals. The cutter is typically held on the bearing pin by a series of ball bearings riding in a race located generally between the two sets of roller bearings. The balls are inserted into the race after assembly of the cutter on the bearing pin.
The ability to withstand lateral loads on the bearing pins is to some extent dependent upon the load-carrying capacity of the two sets of roller bearings. In general, the greater the length of each individual roller, the greater the load-carrying capacity of the roller bearing. The limit of the combined lengths of the two sets of roller bearings is determined by the available length of bearing races, which depends upon the overall length of the bearing pin, allowing for the length required by the ball bearings and the length required by the thrust absorbing means. As the weight on the bit increases, the required load-carrying capacity of the roller bearings increases. Therefore, the length of the roller bearings can be a limiting factor on the allowable weight on bit. Inadequate roller length can cause reduced load-carrying capacity, resulting in reduced bearing life and reduced bit life. It would be desirable to increase the allowable roller bearing length for a given size of cutter, thereby increasing the load-carrying capacity of the bearings.